The role of floodplains for denitrification along large rivers in central Europe – measurements and modeling for a comprehensive overview

Floodplains, especially along large rivers in central Europe, have been modified heavily during the last centuries. Not only floodplain losses by embankment and dikes, but also a decoupling of river hydrology by river bed incision and embankments occurred. Nevertheless, floodplains are known to act as nutrient sinks when inundated by river water during floods. Nutrients, such as nitrate, are nowadays known to be of higher concentrations than under natural conditions and can cause water quality issues. How much of this retention function is left in several highly modified European rivers?  

 

With this study, we provide an overview of our activities to quantify denitrification along large rivers in Germany and the Danube in Austria as well as in its river basin by applying models as well as field and laboratory measurements. We explore this key ecosystem function of endangered floodplain ecosystems and their potential to act as nature-based solutions to mitigate the effects of nitrate pollution.

Therefore, we modeled denitrification by a semi-empiric model for the rivers Rhine, Elbe, Main, and Weser as well as the Danube. For the latter, a comparison with a statistical model based on in-site measurement on nutrient concentrations was carried out. Furthermore, we are currently estimating the denitrification potential of the Danube River by applying the semi-empirical model for in-stream retention and for floodplains, considering controlling soil physical and chemical parameters as well as flooding probabilities.

On the measurement side, we applied the acetylene inhibition method for 6 locations (113 plots) along the rivers Elbe, Rhine, Main, and Weser together with calculated average inundation time to determine upper-bound soil denitrification potential estimates. Through this combined model- and measurement-based approach, proxy-based statements were further developed on a large scale.

From all our studies we conclude that floodplains still contribute to nitrate retention in large modified rivers. Depending on the level of connectivity between rivers and their adjacent floodplains, the occurrence of frequent inundation during the year, and the way floodplains are inundated (large spatial extent vs preferential pathways carrying the water quickly through the floodplain), averaged modeled N-retention is about 400 kg N/ha/yr which is mid to upper range of reported removal rates in other systems. However, compared to the large nutrient loads and the small regular inundation extent, current floodplains alone limited in their current connectivity level are not capable to improve water quality significantly. Based on the laboratory measurements resulting in lower retention rates of 25-100 kg N/h/yr we conclude that the pH value is a key parameter that influences denitrification: thus, even in well-connected floodplains denitrification potential is comparably low due to low pH values as proved for the Elbe River. Further field studies and laboratory experiments under closer in-situ conditions are necessary to better understand which processes limit and foster denitrification. Therefore, we adapt our methods to assess actual/quasi-in-situ soil denitrification on a study site along the Havel River in spring 2022.